In nuclear reactors, for example, a boiling water reactor, nuclear fuel rods are grouped together in an open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. The fuel rods are supported between upper and lower tie plates in side-by-side parallel arrays. Spacers are employed at predetermined elevations along the fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation.
A typical spacer for this purpose is disclosed in U.S. Pat. No. 5,002,726, of common assignee herewith. In that spacer, ferrules are arranged in a matrix, with each ferrule comprising a generally cylindrical element secured to an adjacent ferrule and having a generally C-shaped slot in the ferrule wall in registration with a corresponding slot in an adjoining ferrule. A closed-loop spring is disposed about the tabs of the C-shaped openings of adjoining ferrules and has medial portions projecting into the cylindrical spaces defined by the ferrules for engaging nuclear fuel rods within the adjacent ferrules and biasing the rods against stops adjacent opposite sides of the ferrules.
While this ferrule arrangement has proven satisfactory, there are problems associated with springs and ferrules of this and other types. For example, the coolant flow in a nuclear fuel rod bundle is upward and consists of a steam/water mixture. High-velocity water droplets impinge on the bottom of looped springs of this type and may cause erosion of the spring material. Further, looped springs of this type are comparatively expensive to manufacture. The ferrules similarly are expensive to manufacture because of the complex shape of the side cutout and tab necessary to mount the spring. Additionally, "soft assembly" is difficult with the spring ferrule arrangement. "Soft assembly" is a process in which fuel rods having a coating or protective surrounding sleeves are inserted into the spacers. The presence of a coating or sleeve requires clearance between the bulbous end portions of the looped springs and the fuel rod and this is difficult to obtain with these looped springs.